Ternary oxide phosphor particles

ABSTRACT

Phosphor compositions are prepared by treating metal oxides or mixed-metal oxides with refractory metals to form cathodoluminescent phosphors stimulatable by electrons of very low energy. The phosphors comprise 90% to 100% of a mixed metal oxide Mx,TyOz (where M is a metal selected from Zn, Sn, In, Cu, and combinations thereof; T is a refractory metal selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and combinations thereof; and O is Oxygen, x, y, and z being chosen such that z is at most stoichiometric for MxTyOz) and 0% to 10% of a dopant comprising a substance selected from a rare earth element of the lanthanide series, Mn, Cr, and combinations thereof, or stoichiometrically excess Zn, Cu, Sn, or In. A blue-light-emitting phosphor based on ZnO treated with Ta2O5 or Ta to form Ta2Zn3O8 is characterized by CIE 1931 chromaticity values x and y, where x is between about 0.14 and 0.20 and y is between about 0.05 and 0.15. In preferred embodiments, a process is specially adapted for forming the phosphor in an electrically-conductive thin-film or surface-layer form in situ during fabrication of displays. A preferred in situ process has an integrated etch stop, which precisely defines the depth of an opening in a field-emission display structure utilizing the low-energy-electron excited phosphor. A field-emission display comprises cells, each having a field-emission cathode and an anode comprising at least one cathodoluminescent phosphor. Arrangements of various color phosphors may be made by selective deposition of suitable dopants. The display cell structures may also have gate elements for controlling electron current flowing to the anode and its phosphor when suitable voltages are applied.

RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. §371 of PCTInternational application PCT/GB99/04300, filed Dec. 17, 1999, which waspublished under PCT Article 21(2) in English. Foreign priority benefitsare claimed under 35 U.S.C. §119(a)-(d) or 35 U.S.C. §365(b) of GreatBritain application number GB 9911781.4, filed May 20, 1999 and GreatBritain application number GB 9827859.1, filed Dec. 17, 1998.

The present invention relates to rare earth activated phosphors. Suchphosphors are known to possess excellent light output and colourrendering properties and have been utilized successfully in many displaytechnologies. One particularly successful material, europium activatedyttrium oxide (Y₂O₃:Eu³⁺), has shown particular promise in the field offield emission display.

The successful introduction of field emitting displays is dependent uponthe availability of low voltage phosphors. As the phosphor excitingelectrons have a comparatively low energy (less than 2 kV) as comparedto conventional phosphors and one must avoid the use of sulphur toreduce contamination, new types of material have to be used. Inparticular, it is desirable to be able to make phosphor particleswithout a surface dead layer which occurs when fine particles areprepared using a conventional grinding technique. This dead layer is animportant source of non-radiative luminescence routes for low energyelectrons.

It is known that collodial chemical techniques may be used to providesub 100 nm particles of compounds such as Y₂O₃ and that these may bedoped to form nanocrystalline red emitting Y₂O₃:Eu³⁺. However, binaryoxide materials such as Y₂O₃ and Gd₂O₃ are not efficient hosts forelements other than europium. In particular, they cannot be used as hostmaterials for blue emitting cerium based phosphors.

Accordingly, there is the need to obtain small, typicallynanocrystalline, particles which will provide different emissioncolours.

It has now been found, according to the present invention, that as wellas red emitting particles, other coloured emitting particles can beobtained when using as a host a ternary oxide, that is to say an oxidewhich is derived from another element apart from yttrium, gallium etc.Accordingly the present invention provides particles of a compound offormula:

Z_(z)X_(x)O_(y):RE

where Z is a metal of valency b

X is a metal or metalloid, of valency a, such that

2y=b.z+a.x, and

RE is a dopant ion of terbium, europium, cerium, thulium, samarium,holmium, erbium, dysprosium, praseodymium, manganese, chromium ortitanium, having a size not exceeding 1 micron.

It will be appreciated that X must be such as will be capable of formingan anion with oxygen.

Generally the particles are nanoparticles, by which is meant particleshaving a size not exceeding 100 nm, generally not exceeding 50 nm andespecially not exceeding 30 nm.

Z is typically trivalent or pentavalent and is preferably yttrium,gadolinium, gallium or tantalum with yttrium being particularlypreferred. X is generally divalent or trivalent, preferably aluminium,silicon or zinc, with aluminium particularly preferred.

The rare earth element is preferably europium, terbium, cerium, thuliumor dysprosium.

Particular particles of the present invention are those derived fromyttrium and aluminium, yttrium and silicon, tantalum and zinc or zincand gallium.

Specific compounds of the present invention includeY₃Al₅O₁₂:Tb³⁺(referred to as YAG:Tb), Y₂SiO₅:Ce³⁺, Ta₂Zn₃O₈ and ZnGa₂O₄.

Thus the particles of the present invention can be green emitting aswith Y₃Al₅O₁₂:Tb³⁺ or blue as in Y₂S_(i)O₅:Ce³⁺ although green emissionmay also be obtained from the yttrium aluminium compound if theconcentration of terbium is reduced. Other colours can be obtained fromother specified rare earth elements, for example, as follows:orange-samarium, blue-holmium, near infra-red-erbium orwhite-disprosium.

In general, the particles of the present invention are in the form ofsingle crystals.

The particles of the present invention can generally be prepared by thecoprecipitation of salts of the two metals (for simplicity X will bereferred to hereafter as a metal) of, the ternary oxide and of the “rareearth” element in aqueous solution at elevated temperature which is thencalcined to the oxide. According to the present invention there isprovided a process for preparing the particles of the present inventionwhich comprises:

preparing an aqueous solution of salts of Z, X and RE and awater-soluble compound which decomposes under the reaction conditions toconvert said salts into hydroxycarbonate,

heating the solution so as to cause said compound to decompose,

recovering the resulting precipitate and

calcining it at a temperature of at least 500° C.

This process is similar to that disclosed in GB Application No.9827860.9 The water-soluble compound which decomposes under the reactioncondition is typically urea, which is preferred, or a weak carboxylicacid such as oxalic acid or tartaric acid. The urea and other watersoluble compounds slowly introduce OH⁻ ligands into the solution untilthe solubility limit has been reached. When the urea decomposes itreleases carbonate and hydroxide ions which control the precipitation.If this is done uniformly then particles form simultaneously at allpoints and growth occurs within a narrow size distribution.

The nature of the salts of the metals is not particularly criticalprovided that they are water soluble. Typically, the salts arechlorides, but, for instance, aluminium perchlorate can also be used.

The reaction is carried out at elevated temperature so as to decomposethe water soluble compound. For urea, the lower temperature limit isabout 70° C.; the upper limit of reaction is generally 100° C.

The relative amounts of the two metal salts should be such as to providethe appropriate ratio of the metals in the mixed oxide. This can, ofcourse, be found by simple experiment. Careful control of the relativeamounts can be important as there is a tendency for the compounds toform a number of phases.

Doping with the “rare earth” metal salt can be carried out by adding therequired amount of the dopant ion, typically from 1 to 10%, for exampleabout 5% (molar).

The reaction mixture can readily be obtained by mixing appropriateamounts of aqueous solutions of the salts and adding the decomposablecompound.

It has been found that rather than start the process by dissolving saltsof the desired elements there are advantages to be obtained by preparingthe salts in situ by converting the corresponding oxides to these salts.Apart from the fact that oxides are generally significantly cheaper thanthe corresponding chlorides or nitrates, it has also been found that thecathodoluminescence of the resulting particles can be superior.

It has been found that better results can generally be obtained bykeeping the reaction vessel sealed. This has the effect of narrowing thesize distribution of the resulting precipitate.

An important feature of the process is that decomposition takes placeslowly so that the compounds are not obtained substantiallyinstantaneously as in the usual precipitation techniques. Typically forurea, the reaction is carried out at, say, 90° C. for one to four hours,for example about 2 hours. After this time precipitation of a mixedamorphous/nanocrystalline phase is generally complete. This amorphousstage should then be washed and dried before being calcined.Decomposition of urea starts at about 80° C. It is the temperature whichlargely controls the rate of decomposition.

Calcination typically takes place in a conventional furnace in air butsteam or an inert or a reducing atmosphere such as nitrogen or a mixtureof hydrogen and nitrogen can also be employed. It is also possible touse, for example, a rapid thermal annealer or a microwave oven. Theeffect of using such an atmosphere is to reduce any tendency the rareearth element may have from changing from a 3+ ion to a 4+ ion. This isparticularly prone in the case of terbium and cerium as well as Eu²⁺.The use of hydrogen may also enhance the conductivity of the resultingcrystals. Calcination generally requires a temperature of at least 500°C., for example 600° C. to 900° C., such as about 650° C. It has beenfound that by increasing the calcination temperature the crystallitesize increases. Indeed it is possible to produce monocrystals having alarger particle size by this process. It has also been found that byincreasing the crystallite size of the resulting particles theluminescence of the particles is enhanced.

In general it has been found that grain growth becomes significant oncethe temperature reaches 1000° C. While there is a small improvement incrystallite size when using 900° C. rather than 600° C. this isrelatively insignificant compared with the increases which occur oncethe temperature is raised to 1000° C. or above. In general thetemperature required is from at least one third to half the bulk meltingpoint of the oxide (the Tamman temperature) which is typically of theorder of 2500° C. Best results are generally obtained towards the upperend of this range.

Time also plays a part and, in general, at higher temperatures a shortertime can be used. In general the calcination is carried out at atemperature and time a sufficient to produce a crystallite size of atleast 35 nm, generally at least 50 nm.

The time of calcination is generally from 30 minutes to 10 hours andtypically from 1 hour to 5 hours for example about 3 hours. A. typicalcalcination treatment involves a temperature of at least 1050° C., eg.1050° C. for 3 hours while at lower temperatures a time from 3 to 6hours is typical. In general, temperatures above 1300 to 1400° C. arenot needed. In order to augment crystallite size it is possible toincorporate flux agents which act as grain boundary promoters such astitania, bismuth oxides, silica, lithium fluoride and lithium oxide.

While, in the past, using a lower temperatures of calcination,crystallite sizes of the order of 20 nm are obtained it has been found,according to the present invention, that crystallite sizes of at least50 nm are regularly obtainable. Indeed crystallite sizes as much as 200nm can be obtained without difficulty. As the temperature of calcinationincreases the particles have a tendency to break up into single ormonocrystalline particles. If the calcination takes place for too longthere is a danger that the particles will not disperse. Obviously theparticle size desired will vary depending on the particular applicationof the phosphors. In particular the acceleration voltage affects thesize needed such that at 300 volts a crystallite size of the order of 50nm is generally suitable.

The urea or other decomposable compound should be present in an amountsufficient to convert the salts into hydroxycarbonate. This means thatthe mole ratio of eg. urea to salt should generally be at least 1:1.Increasing the amount of urea tends to increase the rate at whichhydroxycarbonate is formed. If it is formed too quickly the size of theresultant particles tends to increase. Better results are usuallyobtained if the rate of formation of the particles is relatively slow.Indeed in this way substantially monocrystalline particles can beobtained. In general the mole ratio of urea or other decomposablecompound to salt is from 1:1 to 10:1, for example from 1:1 to 8:1, say2:1 to 6:1 or 2:1 to 5:1, for example about 3:1 although higher ratiosmay be desirable if the initial solution is acidic and sometimes theyimprove yield. Typically the pH will be from about 0.5 to 2.0 althoughsomewhat different values may be used if the salt is formed in situ. Ingeneral, the effect of the mole ratio on crystallite size isinsignificant when the calcination temperature exceeds 1000° C.

A variant of this process is needed where the initial salt canprecipitate. Thus in the case of silicon, it is desirable first to forma seed particle of silicon which is then reacted to form a “metal/dopeand shell” around it. Thus silicon tetrachloride, for example, isprecipitated from water to form a hydrated silicon oxide. This colloidalsolution is then added to a solution of the other metal salts and of thedopant salt. The other steps of the process including the addition ofthe decomposable compound can then be carried out as discussed above.Other particles which can be prepared in a similar manner includeTa₂Zn₃O₈ (tantalum oxide particles with a precipitation of zinc aroundit) doped with terbium and manganese for green and red emission andZnGa₂O₄ gallium oxide particles with a zinc precipitate doped withmanganese, terbium, europium or cerium.

The particles of the present invention are suitable for use in FED typedisplays. For this purpose the particles can be embedded in a suitableplastics material by a variety of methods including dip coating, spincoating and meniscus coating or by using an air gun spray. Accordingly,the present invention also provides a plastics material whichincorporates particles of the present invention.

Suitable polymers which can be employed include polyacrylic acid,polystyrene and polymethyl methacrylate. Such plastics materials can beused for photoluminescence applications and also in electroluminescenceapplications where an AC current is to be employed. If a DC current isemployed then conducting polymers such as polyvinylcarbazole,polyphenylenevinylidene and polymethylphenylsilane can be employed. Poly2-(4-biphenylyl)5-(4-tertiarybutyl phenyl)-1,3,4-oxidiazole (butyl-PBD)can also be used. Desirably, thepolymer should be compatible with thesolvent, typically methanol, used to apply the particles to the plasticsmaterial.

Typically, the particles will be applied to a thin layer of the plasticsmaterial, typically having a thickness from 0.5 to 15 microns.

The maximum concentration of particles is generally about 35% by weightwith 65% by weight of polymer. There is a tendency for the polymer tocrack if the concentration exceeds this value. A typical minimumconcentration is about 2% by weight (98% by weight polymer). If theconcentration is reduced below this value then “holes” tend to form inthe plastics material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the luminescence efficiency of nanocrystalline Y₂O₃:Tb³⁺and of nanocrystalline YAG:Tb.

FIG. 2 is a high resolution electron micrograph (scale 1 cm=25 mm) ofYAG:Tb calcined at 650° C.

FIG. 3 shows the cathodoluminescence obtained with YAG:Tb powder atequal loading with MgO binder.

FIG. 4 shows the effect of firing temperature on crystallite size.

The following Examples further illustrate the present invention.

EXAMPLE 1 Synthesis of YAG:Tb

This procedure involves co-precipitation of yttrium, aluminium andterbium in an aqueous solution at elevated temperatures to form 20-30 nmamorphous particles, which are then calcined into the oxide.

(1) 0.15 M solutions of YCl₃ and Al(ClO₄)₃ are mixed in the ratio of15:25 by volume.

To this a 5M urea solution is added to double the volume of theconstituents.

Doping is carried out by adding the required (typically 5%) amount ofthe dopant ion in the form of a 0.15M solution of terbium chloride.

(2) The reaction vessel is sealed and raised to 90° C. in a water bathfor 2 hours. After this time precipitation of the amorphous phase willbe complete.

(3) The amorphous phase is washed several times and dried.

(4) The amorphous phase is then fired in air at a temperature greaterthan 600° C. for three hours. This results in the crystallisation of theamorphous phase into 10-20 nm nanocrystals of YAG:Tb.

The advantage of utilising the ternary host as the carrier for the Tb³⁺ion is demonstrated in FIG. 1 of the accompanying drawings, in which theluminescence efficiency of nanocrystalline Y₂O₃:Tb³⁺ and thenanocrystalline YAG:Tb is shown. The luminescence efficiency of theYAG:Tb phosphor is at least five times that of the binary compound. Theefficiency of Y₂O₃:Eu is about 70% that of YAG:Tb. In FIG. 2 a highresolution electron micrograph (scale 1 cm+25 nm) of the YAG:Tb calcinedat 650° C. is shown; features appearing bright are crystals.

EXAMPLE 2 Synthesis of Y₂SiO₃:Ce

In this Example a seed nanoparticle of silica is first grown, and theyttrium/dopant shell is precipitated around it before firing.

(1) 0.7 ml of SiCl₄ is added dropwise to 40 ml water at 0° C. Thesolution is stirred vigorously during the precipitation. The SiCl₄ formsan hydrated silicon oxide precipitate with the evolution of HCl.

(2) 13 ml of the colloid solution is added to 27 ml 0.15M YCl₃ and 40 ml5M urea at room temperature. The dopant typically 1 to 10% is then addedin the form of 0.15M CeCl₃.

(3) The reaction vessel is sealed and placed in a water bath at 90° C.for two hours.

(4) The amorphous precipitate thus formed is washed, dried and firedunder air at greater than 650° C. for three hours.

Luminesence from the ternary based material occurs in the blue region ofthe spectrum. Cerium does not exhibit luminescence at all if doped intoa binary compound.

EXAMPLE 3

The amorphous phase precipitate of YAG:Tb (Y₃Al₅O₂:Tb³⁺) obtained inExample 1 was washed and dried. It was then fired in air at temperaturesof 650° C., 850° C., 1050° C. and 1150° C. for 3 hours. FIG. 3 shows thecathodoluminescence obtained with powders at equal loading with MgObinder. It can be seen that increasing the calcination to at least 1000°C. results in a significant increase in luminescence. At 1000V there isan increase in CL by a factor of 150 as the temperature is increasedfrom 650° C. to 1150° C.

A concern with terbium-containing compounds is oxidation from 3+ to 4+state, and the quenching behaviour of the 4+ state on luminescence. Acharacteristic of this oxidation is a change of colour of the powderfrom white to yellow or brown. No such colour change was observed. FIG.4 shows how the crystallite size increases correspondingly as the firingtemperature is increased.

What is claimed is:
 1. A process for preparing phosphor particles offormula: Z_(z)X_(x)O_(y):RE where Z is a metal of valency b, X is ametal or metalloid, of valency a, such that 2y=b.z+a.x, and RE is adopant ion of terbium, europium, cerium, thulium, samarium, holmium,erbium, dysprosium, praseodymium, manganese, chromium or titanium havinga size not exceeding 1 micron which comprises preparing an aqueoussolution of salts of Z, X and RE and a water soluble compound whichdecomposes under the reaction conditions to convert said salts intohydroxycarbonate, heating the solution so to cause said compound todecompose, recovering the resulting precipitate and calcining it at atemperature of at least 500° C.
 2. A process according to claim 1 inwhich at least one of the salts is a chloride.
 3. A process according toclaim 1 in which the salts are formed in situ from the correspondingoxides and the corresponding acid.
 4. A process according to claim 3 inwhich the oxide is provided as a colloidal precipitate.
 5. A processaccording to claim 1 in which the said water-soluble compound is urea oroxalic acid.
 6. A process according to claim 5 in which the solutioncontaining the urea is heated to a temperature of 70 to 100° C.
 7. Aprocess according to claim 1 in which the calcination takes place in airor a reducing atmosphere.
 8. A process according to claim 7 in which thecalcination takes place at a temperature of ⅓ to ½ the Tammantemperature.
 9. A process according to claim 8 in which the calcinationtakes place at a temperature of at least 1050° C.
 10. A processaccording to claim 1 in which RE is added in an amount to provide aconcentration of 1 to 10% in the particles.
 11. A process according toclaim 1 in which the heating step is carried out in a sealed vessel. 12.A process according to claim 8 in which the calcination takes place for1 to 5 hours.